[PATCH] x86_64: fix boot time hang in detect_calgary()

if CONFIG_CALGARY_IOMMU is built into the kernel via
CONFIG_CALGARY_IOMMU_ENABLED_BY_DEFAULT, or is enabled via the
iommu=calgary boot option, then the detect_calgary() function runs to
detect the presence of a Calgary IOMMU.

detect_calgary() first searches the BIOS EBDA area for a "rio_table_hdr"
BIOS table. It has this parsing algorithm for the EBDA:

	while (offset) {
		...
		/* The next offset is stored in the 1st word. 0 means no more */
 		offset = *((unsigned short *)(ptr + offset));
	}

got that? Lets repeat it slowly: we've got a BIOS-supplied data
structure, plus Linux kernel code that will only break out of an
infinite parsing loop once the BIOS gives a zero offset. Ok?

Translation: what an excellent opportunity for BIOS writers to lock up
the Linux boot process in an utterly hard to debug place! Indeed the
BIOS jumped on that opportunity on my box, which has the following EBDA
chaining layout:

  384, 65282, 65535, 65535, 65535, 65535, 65535, 65535 ...

see the pattern? So my, definitely non-Calgary system happily locks up
in detect_calgary()!

the patch below fixes the boot hang by trusting the BIOS-supplied data
structure a bit less: the parser always has to make forward progress,
and if it doesnt, we break out of the loop and i get the expected kernel
message:

  Calgary: Unable to locate Rio Grande Table in EBDA - bailing!
Signed-off-by: NIngo Molnar <mingo@elte.hu>
Acked-by: NMuli Ben-Yehuda <muli@il.ibm.com>
Signed-off-by: NLinus Torvalds <torvalds@osdl.org>

arch/x86_64/kernel/pci-calgary.c

@@ -1052,7 +1052,7 @@ void __init detect_calgary(void)
 	void *tbl;
 	int calgary_found = 0;
 	unsigned long ptr;
-	int offset;
+	unsigned int offset, prev_offset;
 	int ret;
 
 	/*
@@ -1071,15 +1071,20 @@ void __init detect_calgary(void)
 	ptr = (unsigned long)phys_to_virt(get_bios_ebda());
 
 	rio_table_hdr = NULL;
+	prev_offset = 0;
 	offset = 0x180;
-	while (offset) {
+	/*
+	 * The next offset is stored in the 1st word.
+	 * Only parse up until the offset increases:
+	 */
+	while (offset > prev_offset) {
 		/* The block id is stored in the 2nd word */
 		if (*((unsigned short *)(ptr + offset + 2)) == 0x4752){
 			/* set the pointer past the offset & block id */
 			rio_table_hdr = (struct rio_table_hdr *)(ptr + offset + 4);
 			break;
 		}
-		/* The next offset is stored in the 1st word. 0 means no more */
+		prev_offset = offset;
 		offset = *((unsigned short *)(ptr + offset));
 	}
 	if (!rio_table_hdr) {